ELI5: Is it pissing in public if you don't have curtains?

Just woke up from a night of drinking and stumbled to my buddy's guest bathroom (crashed at his place) to relieve my bladder. After entering the bathroom, I didn't shut any of the doors and while doing my biz i look through the door to the right and have a clear view of the front window likewise for anyone walking past that window. If someone had walked and looked at me (I'd wave of course) could they call the cops??

It could be. For instance, the law against public lewdness in New York is Penal Law Section 245: "A person is guilty of public lewdness when he intentionally exposes the private or intimate parts of his body in a lewd manner or commits any other lewd act (a) in a public place, or (b) in private premises under circumstances in which he may readily be observed from either a public place or from other private premises, and with intent that he be so observed."

So in NY, it would have to be proved that you were peeing with the intent to be observed to be convicted, which may or may not have applied to your situation. Every state's laws will be different.

If you are on your property you can do what you want. If you are visible from public space that protects people who are looking at you from being sued by you, but it does not grant them the ability to sue you. At least in Texas I could stand naked in my door and be fully legal as I am in my home, but if I stepped out to get the mail I would run the risk of being arrested for indecent exposure.